The PCR for the Long Product.

Sometimes it is difficult to got the long PCR product (such as 5kb), there are many influence facter, such as for the RT-PCR, The quality of the RNA, the Reverse Transcription, the primer, and the procedure of the PCR , and especially the Taq enzyme. but I think usually the Taq enzyme play an important role in this reaction. we need some taq which can amplify the long product, there are many kind of this enzyme.